1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method which form an image by using a print head capable of ejecting a black ink and a plurality of color inks that can describe black by at least color mixing.
2. Description of the Related Art
Because of its high-density and high-speed printing operation, an ink Jet printing apparatus that prints on a variety of kinds of print media has found applications, and been commercialized, as printers and portable printers for printing output media in various equipment.
A common ink jet printing apparatus has a carriage carrying a print head as printing means and an ink tank, a transport means for feeding a print medium, and a control means for controlling these operations. The print head can eject ink droplets from a plurality of nozzles. Such an ink jet printing apparatus forms an image in a print area on the print medium by repeating a printing operation and a print medium feeding operation. The printing operation involves serially scanning the print head in a direction (main scan direction) perpendicular to a print medium transport direction (subscan direction). The print medium feeding operation involves moving the print medium a distance equal to a print width in the subscan direction. This printing method ejects ink onto the print medium according to a print signal to form an image and is widely used thanks to its advantages of a low running cost and a quiet operation. In recent years, various kinds of color ink jet printing apparatus capable of forming color images by using a plurality of color inks have been put to practical use.
The black ink used in such a color ink jet printing apparatus is frequently used for character printing and thus required to provide high level of sharpness, vividness and print density of an image. To meet this requirement, a technique is known to lower a penetrability of black ink into a print medium to retard a colorant in the black ink from soaking in the print medium (for example, Japanese Patent Application Laid-open No. 9-25442).
As for color inks other than black ink, when two different color inks land on a print medium close to each other, the inks may mix together at a boundary between them, degrading a quality of a printed color image, a phenomenon called a boundary bleeding. To prevent this, there is known a technique that enhances a penetrability of color inks into the print medium to prevent the color inks from mixing together on the surface of the print medium (for example. Japanese Patent Application Laid-open No. 55-65269).
The use of a black ink with low penetrability and color inks with high penetrability, however, gives rise to the following two problems.
First, the color inks with high penetrability have a short fixing time whereas the black ink with low penetrability takes longer to be dried and fixed. Thus, when a plurality of sheets of print medium are printed successively, with the printed sheets discharged one after another onto the preceding sheets, the subsequently discharged sheet may get smeared by the black ink applied to the preceding sheet and not yet dried completely as its back comes into contact with the printed front surface of the preceding page. This smear resulting from the contact between the front printed surface of the preceding sheet and the back surface of the subsequent sheet becomes prominent as the printing speed increases.
Second, since the black ink has a low penetrability, an ink spreading may occur in boundary portions on the print medium between black ink landing portions and color ink landing portions (this is called a boundary bleeding). This significantly degrades the quality of color image.
To cope with the above two problems, the following three measures have been taken.
A first measure employs an ink fixing means such as a heating/fixing device. This method enables ink to be fixed in the print medium quickly, preventing smear and boundary bleeding (for example, Japanese Patent Application Laid-open No. 7-47762)
A second measure is to perform a sheet discharge wait control. This method involves delaying the start of printing operation on the second sheet until the ink on the first sheet fully dries, or temporarily halting the discharge operation after the second sheet has been printed. This method too can prevent smear (for instance, Japanese Patent Application Laid-open No. 2-303842)
A third measure is to apply highly penetrating color inks so as to overlap those areas where the black ink is to be applied. Applying the black ink to areas of the print medium where color inks are applied allows the black ink to be fixed more easily on the print medium surface, minimizing smear. Further, the boundary bleeding can also be reduced by the use of an ink set which makes the black ink and the color inks react with each other for coagulation (e.g., Japanese Patent Application Laid-open No. 2003-159827).
These first to third measure, however, have the following drawbacks.
The first measure employs a fixing means, so an increase in cost and size of the apparatus is unavoidable Further, since in the serial printer, the print medium feed operation is performed intermittently, when the print medium is passed through the fixing device, there may be variations in how well the printed image is fixed, according to the locations on the print medium.
The second measure degrades throughput since it performs the print medium discharge operation delay control.
Since the third measure applies color inks in a way that overlaps black ink application areas, degradations may result in terms of a sharpness of a black ink image and a printed quality of black characters If the amount of color inks required for the prevention of smear and the amount of color inks required for the prevention of boundary bleeding differ, it is difficult to prevent both of smear and boundary bleeding at the same time.